The present invention relates to the structure of a semiconductor laser. In particular, the invention relates to the structure of a low-noise semiconductor laser having no astigmatism which is suited for a light source for devices for consumer electronics such as compact disk and optical video disc, for optical information processing systems and for optical communications.
In using a semiconductor laser as a light source for devices for consumer electronics such as optical video discs and compact discs, and for optical information processing systems, the light reflected by the optical system in these devices returns to the semiconductor laser, which is the light source, giving rise to the occurrence of such a phenomenon that noise increases in the semiconductor laser. In order to reduce the above optical feedback induced noise, the semiconductor laser must be self-pulsated or the oscillating spectrum must be multiplied. However, the self-pulsating or the spectrum-multiplied (multi-longitudinal-mode operation) laser induces astigmatism which must be corrected by employing a cylindrical lens when the laser is to be adapted to the devices. Such lasers have been taught, for example, in the preparatory document for the Thirty-First Association of the Applied Physics, p. 177, 30a-M-6 and 30a-M-8 and in the preparatory document for the Forty-Fourth Association of the Applied Physics, p. 110, 27a-p-3. However, the former laser does not self-pulsate, and the latter laser induces astigmatism though it self-pulsates.
FIG. 1 shows a conventional semiconductor laser obtained by the metal organic chemical vapor deposition (MOCVD) method.
The semiconductor laser of this structure has been disclosed in J. J. Coleman et al., Applied Physics Letters, Vol. 37, 1980, p. 262. This laser is so constructed that a striped n-type injection current confinement layer 5 is formed on cladding layers 2, 4 between which an active layer 3 is placed, and the injection current confinement layer 5 is buried under a cladding layer 6 and a cap layer 7 to control the lateral mode. Reference numerals 8 and 9 denote a p-type electrode and an n-type electrode respectively. With the semiconductor laser of this structure, the mode control is of the type of index-guiding, and the longitudinal mode is a single mode. Therefore, the level of relative intensity noise (RIN) is about 10.sup.-12 H.sub.z.sup.-1 (when the light is fed back from the optical system), and the noise must be reduced by adopting a high-frequency superposition system or a like system.